Before you take another probiotic cap, you may want to read this. Yet another study at McMaster University in Canada suggests that gut bacteria might be able to alter your brain chemistry and change your mood and behavior, reports Science NOW.

We reported on earlier research on gut bacteria at McMaster University and at Ohio State University. We also mentioned some heady speculations at Texas Tech University Health Sciences Center on a new field of microbial endocrinology, “where microbiology meets neuroscience.”

In the new study, McMaster researchers take a slightly bolder step. They fed mice a broth containing a “benign” bacterium, Lactobacillus rhamnosus. The scientists chose this partly because, well, they had some around, so why not, and also because related Lactobacillus bacteria are a major ingredient of probiotic supplements and very little is known about their potential side effects.

How it works, maybe

They found that mice whose diets were supplemented with L. rhamnosus for 6 weeks exhibited fewer signs of stress and anxiety in standard lab tests, as reported online in the Proceedings of the National Academy of Sciences. For example, the mice spent more time exploring narrow elevated walkways and wide-open spaces, which are scary to rodents. (That might explain the Jackass movies?)

But how does it work? That’s the part that has been puzzling me. First, the McMaster researchers found changes in the activity of genes in the brain that encode portions of the receptor for the neurotransmiter GABA. GABA typically dampens neural activity, and many drugs for treating anxiety disorders target its receptors. (The Texas Tech guys also suggested GABA effects.)

Then they cut the vegas nerve, which runs between the stomach and brain, and lo and behold, the effects stopped. The findings “open up very exciting speculations” about using probiotics to treat mood disorders in people, says Emeran Mayer, a gastroenterologist at the University of California, Los Angeles. But he said he’s skeptical that the findings will translate easily from mice to people.